Rheology of Tire Rubber

The Challenge

Tires are manufactured worldwide for cars, trucks, industrial vehicles, and common conveyances such as baby carriages, bicycles etc. A tire is a strong and flexible rubber casing attached to the rim of a wheel that provides a gripping surface for traction and serves as a cushion for the wheels. Natural or synthetic rubber are typically the primary material use in tire production however, thermoplastic elastomers are often used to produce tires. Thermoplastic elastomers have the same mechanical and chemical properties, but can also be easily recycled and processed via extrusion and injection molding techniques (common methods used for thermoplastics). A tire manufacturer needed to study their rubbery compound for its viscosity properties and swelling at the exit of the die. The sample consisted of an elastomer, carbon black, and additives in form of flat sheets.

Our Solution

A capillary rheometer is a useful tool for determining flow characteristics of thermoplastic elastomers to understand their behavior during the processing phase. Besides determining the rheological curve (viscosity vs shear rates), it is often useful to study the extrudate swelling characteristics thanks to an add-on laser device for swell measurements. A CEAST SR20 (equipped with a 20 kN load cell) is used to perform both rheological tests according to ISO 11443 and die swell tests. The rheological test was carried out at 100°C spanning shear rates between 1 to 1000 s-1 through a capillary die and a twin bore configuration was used to perform two tests simultaneously. This material showed a non-Newtonian behavior with a viscosity ranging from 100,000 Pa*s at 1 s-1 shear rate to 800 Pa*s at 1000 s-1. Overall, the sample showed good repeatability and reproducibility of results. Furthermore, a die swell test was carried out by using the CEAST die swell laser system to study the influence of swell in this compound upon exit from the die.